Ryanair did not supply the wheelchair. BAA did not provide the wheelchair. The wheelchair was supplied by a third party service provider who charged Ryanair £18 for the service. Ryanair, having taken £10 from the customer in fares, passed the wheelchair charge on to the customer.
I don’t support Ryanair, but do understand that they feel they have a case for arguing that BAA should have paid for this service.